This invention relates to a thermal cutout fuse or thermal switch which cuts off the electric continuity between a pair of lead wires when the temperature thereof reaches a prescribed unsafe level.
Generally, among the thermal cutout fuses of the class using a thermally sensitive pellet which have been suggested to date, those which exhibit good thermal sensitivity characteristics, provide a reliable action to cut off the electric circuit and rely on combined use of a thermally sensitive pellet and a mechanical spring have been preponderant in number.
They are operated by a general principle that the electric continuity is established and retained by having a stationary contact and a movable contact disposed in an intimately joined state within a fuse housing, keeping the movable contact energized constantly in the direction of departing from the stationary contact by means of a mechanical spring and, at the same time, preventing directly or indirectly the movable contact from moving away from the stationary contact by means of a thermally sensitive pellet which is solid and assumes a fixed volume at temperatures below the prescribed unsafe level. In the normal condition, therefore, the two contacts are held in intimate contact with each other and the pair of lead wires terminating into the contacts are allowed to retain electric continuity therebetween. When the temperature of the immediate ambience rises beyond the prescribed unsafe level, the thermally sensitive pellet instantaneously melts and liquefies and, consequently, yields to the energizing force of the mechanical spring, with the result that the movable contact is slid away from the stationary contact and the electric continuity between the two lead wires is cut off.
In the thermal cutout fuse formerly proposed by the inventor (Japanese Published Unexamined patent application No. 42640/1979), the movable contact is slid inside the housing in the direction perpendicular to the plane of contact between the movable contact and the stationary contact. At that time, the peripheral surface of the movable contact is rubbed against the inner wall surface of the housing. When the frictional force generated between the two surfaces varies, even if very slightly, from one fuse housing to another, there may be times when the movable contact will be obstructed from generating a sliding motion or it will be caused to slide in a slanted posture, with the result that the departure of the movable contact from the stationary contact will become incomplete. To ensure that the movable contact produces a smooth sliding motion in case of an emergency, it becomes necessary to use a relatively larger mechanical spring capable of generating a higher energizing force than would otherwise be normally required or to provide the main mechanical spring with an auxiliary spring adapted to prevent the main spring from assuming a slanted posture during its sliding motion. These measures tend to complicate the construction of the fuse, add to the size of the fuse and raise the cost of the fuse.